The conditional multiplexer of the digital communication system disclosed in the above referenced commonly assigned published international application WO 88/04869 outputs a particular input bitstream as part of a candidate output bitstream formed from a plurality of input bitstreams in accordance with the result of an operation performed by processing means and which consists in calculating an estimated output bandwidth of said candidate output bitstream from the mean values of the probability distribution functions of the bitrates of said input bitstreams and in the subsequent comparison of said estimated output bandwidth with the maximum allowable output bandwidth or a function thereof.
In this known system, each time a path has to be set up towards an output terminal of a switching module for the transmission of an additional input bitstream or input packet stream, a path set up packet containing the mean value of the variable bitrate of this stream is supplied to the switching module. This path set up packet is the first of the input packet stream and precedes the data packets thereof. The latter will only be transmitted after the path set up packet has determined a path and this transmission will occur along this path. The processing means included in the switching module then calculates the estimated output bandwidth by adding the newly received mean value to the previously registered sum of the mean values of the input bitstreams already multiplexed on this output terminal and checks if this estimated bandwidth is below a predetermined percentage, e.g. 80%, of the maximum allowable bandwidth on this output terminal before allowing the path set up packet and later the other data packets of the additional input packet stream to be multiplexed on the output terminal.
It has been found that this known conditional multiplexer may give rise to a relatively high bit loss, apparently because the calculated estimated output bandwidth used to decide on the multiplexing of the input bitstreams is not sufficiently accurate.